1. Field of the Invention
This invention relates to an improved two-wheeler steering device having a rotary type steering damper.
2. Description of the Related Art
As a steering device for steering the front wheel of a two-wheeler (motorcycle), Japanese Utility Model Publication No. Sho 47-17526 proposes a device equipped with a rotary type steering damper.
According to this proposal, when deflection such as a kickback, that is, disturbance, is input to the front wheel of a two-wheeler during traveling, this deflection can be restrained by the damper described above, to prevent manipulation of the handlebar from becoming unstable.
In the above-mentioned proposed technique, however, the damping force generated by the steering damper depends solely on an orifice for controlling the damping force according to a flow rate of a working fluid. Thus, regarding steering operation for two-wheelers, it is impossible for the above-mentioned proposed technique to provide optimum damping characteristics in conformity with the traveling conditions.
It is an object of the present invention to provide a steering device, which is capable of providing optimum damping characteristics for steering operation during traveling of a two-wheeler.
Another object of the present invention is to provide a steering device which is capable of preventing generation of an excessive damping force which would lead to a heavy handlebar manipulation.
In order to attain the above objects, a steering device for a two-wheeler, according to the present invention includes: a frame of a vehicle body; a support member for a front wheel; a steering shaft connecting the support member to the frame so as to be rotatable about an axis thereof; and a rotary type steering damper providing a hydraulic resistance force no matter in which direction the support member rotates with respect to the frame. The steering damper has a vane adapted to swing in synchronism with rotation of the support member, oil chambers defined on both sides of the vane and filled with working fluid, a flow passage causing working fluid to flow from one oil chamber to the other oil chamber as the vane swings, and a damping valve arranged in a flow passage for the working fluid, and an opening of the damping valve is varied by a signal from outside to vary a damping force generated.
Further, according to another aspect of the invention, in the steering device, a fail-safe valve is arranged in the flow passage for the working fluid so as to be in parallel with the damping valve, the damping valve is constructed of a proportional electromagnetic valve adapted to be closed upon extinction of exciting current, and the fail-safe valve is constructed of an electromagnetic valve adapted to be closed upon application of exciting current and to be opened by a resilient force upon extinction of exciting current.
Further according to yet another aspect of the invention, in the steering device, a relief valve is arranged in the flow passage for the working fluid so as to be in parallel with the damping valve, the damping valve is constructed of a proportional electromagnetic valve adapted to be closed upon an increase in exciting current, and the relief valve is adapted to be closed by a resilient force, the relief valve being opened upon sensing a hydraulic pressure when the damping valve is closed.